Pyrolysis gasoline—the C5 to C10 hydrocarbon product fraction—consists mainly of aromatics. The non-aromatics are mainly unsaturated hydrocarbons with a high portion of acetylenes and dienes. This stream is unstable and cannot be stored, as the unsaturated components react further, forming polymers and gum. Depending on the downstream processes pyrolysis gasoline is hydrogenated and fractionated in different steps. The most common process route is as follows:    1. Selective hydrogenation of the total gasoline to hydrogenate acetylenes, dienes, and styrene to olefinic compounds. After stabilization and removal of oil, this stream is suitable for use as motor fuel. The reaction is typically controlled in a way that the residual styrene value is ca 0.5%.    2. Fractionation of the effluent of the 1st stage hydrogenation into a C5 cut, a C6−C8 heart cut, and a C9+ cut.    3. The C6−C8 cut is further processed in a 2nd stage hydrogenation step to convert olefins to paraffins and naphthenes and to convert all sulfur to H2S, which is removed from the product in a downstream stripper. This process is necessary to prepare the heart cut for aromatics recovery. The specification is controlled via the bromine number, which is typically 0.5.
Another (commonly applied) method is a first stage hydrogenation, followed by a fractionating through distillation. FIG. 1 schematically illustrates a system for fractionating a hydrocarbon stream by successive distillation.
As shown in FIG. 1, a hydrogenated (from 1st stage) stream of pygas (101) is first sent to debutanizer column (C-101), splitting the stream in a C4− cut (102) and a C5+ cut (103). The C5+ cut is sent to depentanizer column C-111, where a C5 fraction (104) is recovered and a C6+ fraction (105). The C6+ fraction is sent to dehexanizer column (C-121), where a C6 fraction is recovered (106) and a C7+ fraction (107) is sent to column C-131, splitting it in a gasoline stream (108) and a heavies stream (109).
The C6 fraction (106) is rich in benzene and can be sent to a benzene extraction plant. It is important for this process that the C6 fraction is free from toluene, since this will be co extracted with the benzene and methyl-cyclohexane.
The separation of hydrocarbon streams consumes a large amount of energy. In particular, it has been calculated that the dehexanizer column has an especially high energy consumption. The result of a computer simulation for the process as shown in FIG. 1 is summarized in Table 1. Mass flow is measured in tons per hour.
TABLE 1Stream No.101102103104105106107108109Mass Flow70.50.869.710.758.931.227.813.614.2(t/h)Composition (weight fraction)C4−0.0100.8670.0000.0030.0000.0000.0000.0000.000C50.1640.1330.1650.9070.0290.0550.0000.0000.000C60.3660.0000.3710.0480.4290.7950.0180.0370.000AromaticsC6 Non-0.0690.0000.0700.0420.0750.1400.0020.0040.000aromaticsC70.1630.0000.1640.0000.1940.0000.4130.7660.075AromaticsC7 Non-0.0200.0000.0200.0000.0240.0080.0410.0800.004aromaticsC80.1190.0000.1200.0000.1420.0000.3010.0890.504AromaticsC8 Non-0.0040.0000.0040.0000.0050.0020.0090.0150.002aromaticsC9+0.0850.0000.0860.0000.1020.0000.2170.0090.415
The calculated heat required for distillation is as follows:    Column; Condenser pressure; reboiler duty; condenser duty    C-101: 7.9 bar atmospher (bara); 2068 kilowatts (kW); 1815 kW    C-111: 2.2 bara; 249 kW; 2002 kW    C-121: 2.4 bara; 8046 kW; 7635 kW    C-132: 0.2 bara; 382 kW; 1661 kW
It can be understood that the dehexanizer column (C-121) is the largest energy consumer in this plant, with a heat requirement more than all the other column reboilers of the pygas section together.
Accordingly, it can be desirable to provide a method for separating a hydrocarbon stream between C6 and C7 which is energy efficient.